When scientists talk about the cryosphere, they mean the places on Earth where water is in its solid form, frozen into ice or snow. Read more ...

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The Arctic has gained a prominent role in the climate change debate. Largely due to feedbacks involving changes snow and sea ice cover, the effects of atmospheric greenhouse gas loading are expected to be seen first and be most pronounced in this region. Accelerated Arctic warming through the 21st century is a common feature of climate model projections.

Over the past 100 years, the Arctic has shown significant variability. Strong warming from about 1920 to 1940, not seen in lower latitudes, was followed by cooling up to about 1970, then a strong warming trend extending through the present. The recent warming is larger than for the globe as a whole. It has been attended by significant reductions in sea ice extent. There are also indications of increased precipitation, warming of soils, and changes from tundra to shrub vegetation.

The early 20th century warming appears to have involved an increase in poleward oceanic heat transport in the North Atlantic and reductions in ice extent. This reflects natural climate variability - potential human impacts were fairly small at this time. The recent warming is strongly linked to large-scale changes in the atmospheric circulation, characterized by general upward trends in indices of the Arctic Oscillation (AO) and its close counterpart, the North Atlantic Oscillation (NAO).

The AO/NAO are natural modes of atmospheric variability. Hence, at face value, it would seem that the recent warming could also be explained by natural variability. Due to retreat of the sea ice cover, climate models project the largest temperature changes during autumn over the Arctic Ocean. But observations show larger warmings in winter and spring over sub-arctic lands, associated with the AO/NAO trend. Are the climate models wrong? Probably not.

Statistically removing the amplifying effects of the AO/NAO still leaves a residual warming. And the AO/NAO trend may not be entirely natural. While links with changes in the stratosphere have been proposed, including a role of ozone losses, recent work suggests that greenhouse gas loading has led to increased sea surface temperatures in the tropical Indian Ocean, which have helped to "bump" the AO/NAO toward a postive state. This argues that the strong Arctic warming associated with the AO/NAO is an "indirect" greenhouse signal. Finally, closer inspection of temperature records shows that there is indeed strong autumn warming over those parts of the Arctic Ocean where sea ice losses have been most pronounced. Just as the climate models predict.